Vent holes are often provided in housings of various devices, including: automotive electrical components such as headlamps, rear lamps, fog lamps, turn lamps, motors, various pressure sensors, and pressure switches; cameras; video devices; information terminals such as mobile phones; home electric appliances such as electric shavers and electric toothbrushes; ink cartridge members mounted in copying machines; and lamps for outdoor use. The main purpose of the provision of a vent hole in a housing of a device is to establish communication between the inside and outside of the device and thereby prevent the internal pressure of the device from excessively increasing as the temperature inside the housing rises due to operation of the device. Battery cases are provided with a vent hole far the purpose of ejecting a gas generated during battery operation.
An air-permeable filter may be attached to a vent hole of a housing of a device to prevent water, dust or the like from entering the housing through the vent hole. A porous membrane of a polyolefin resin or fluorine resin is generally used as the air-permeable filter. In particular, a porous membrane produced by stretching of polytetrafluoroethylene (referred to as “Pain” hereinafter) and having a fine porous structure is known to serve as an air-permeable filter exhibiting excellent water repellency and air permeability. Air-permeable filters may have contact with sebum, a surfactant, an oil, or an ink, depending on the environment in which the filters are used. Even with the use of a stretched porous PTFE membrane having excellent water repellency as an air-permeable filter, a liquid with a low surface tension cannot be prevented from penetrating through the air-permeable filter. Thus, air-permeable filters may, depending on their intended use, be subjected to oil-repellent treatment using a treatment agent containing a fluorine-containing polymer.
It is well known that fluorine-containing polymers having a linear perfluoroalkyl group (referred to as “Rf group” hereinafter) are suitable for imparting oil repellency. Fluorine-containing polymers having a Rf group are used as treatment agents for oil-repellent treatment.
It is also known practice to impart oil repellency to an air-permeable filter using a treatment agent as described above in combination with another treatment agent. For example, Patent Literature 1 discloses treating an air-permeable filter with a treatment agent that contains a fluorine-containing polymer having a group and that further contains a fluorine resin having a fluorinated alicyclic structure in the main chain.